FIG. 8 is a block diagram showing an example of a conventional AGC circuit based on a peak detection system. Especially, the AGC circuit based on a peak detection system shown in FIG. 8 is an example of an AGC circuit based on a lower-side peak detection system with single AGC filter, which is used for processing of a VIF signal in a conventional television.
In FIG. 8, the conventional AGC circuit based on a peak detection system comprises a voltage-controlled amplifier 1, a wave detector 2, a voltage comparator 3, and an AGC filter 501. The voltage-controlled amplifier 1 is a circuit which makes it possible to change a gain of an amplifier by a control voltage and amplify an inputted modulated signal with the gain, in which a gain is set to be larger especially when inputted control voltage is higher.
The wave detector 2 is a circuit which demodulates a modulated signal outputted from the voltage-controlled amplifier 1 and outputs a lower-side envelope of the inputted modulated signal as a detection output. The voltage comparator 3 is a circuit which receives a detection output from the wave detector 2 and compares a voltage indicated by the inputted detection output to a prespecified comparison voltage Vc.
The voltage comparator 3 outputs, when the detection output is higher than the comparison voltage Vc as the result of comparison, a logic level signal "H" and outputs a logic level signal "L" when the detection output is lower than the comparison voltage Vc. The comparison voltage Vc is set to be lower than a DC voltage of a detection output in order to detect lower-side peaks.
The AGC filter 501 comprises a switch 4, a constant current source 5, a constant current source 6, and a capacitor 7. The logic level signal outputted from the voltage comparator 3 as the result of comparison is inputted into the AGC filter 501 as a control signal for the switch 4.
The switch 4 selects either one of the constant current source 5 and constant current source 6 based on the signal indicating the above-described result of comparison and connects the selected one to the capacitor 7. More specifically, as shown in FIG. 8, one edge of the capacitor 7 is grounded and the other edge thereof is connected to either one of the constant current source 5 and the constant current source 6. Assuming that the signal showing the above-described result of comparison is a signal A, when the signal A has the logic level "H", the other edge of the capacitor 7 is connected to the constant current source 5 which contributes to current-feed to the capacitor 7, and when the signal A has the logic level "L", the other edge of the capacitor 7 is connected to the constant current source 6 which contributes to drawing in current from the capacitor 7.
In other words, when the detection output is higher than the comparison voltage Vc, the capacitor 7 is recharged by the constant current source 5, and the capacitor 7 is discharged by the constant current source 6 when the detection output is lower than the comparison voltage Vc.
A change in a voltage due to recharging or discharging of the capacitor 7 is inputted into the voltage-controlled amplifier 1 as a control voltage to enable controlling of a gain of the voltage-controlled amplifier 1, and a negative-feedback loop is formed in the entire AGC circuit thereby.
In this process, a charging current by the constant current source 5 is fixed smaller and a discharging current by the constant current source 6 is fixed larger. With this fixation, when a detection output is lower than a comparison voltage, the gain of the voltage-controlled amplifier 1 is quickly decreased to operate so that the detection output becomes Vc. On the other hand, when the detection output is higher than the comparison voltage, the gain of the voltage-controlled amplifier 1 hardly changes, and the lower-side peak detection becomes possible.
The AGC circuit based on a lower-side peak detection system is formed by the negative-feedback loop as described above. Because of this configuration, a lower-side peak potential of the detection output converges to be coincident with the comparison voltage Vc of the voltage comparator 3.
However, when it is tried to quicken the operation speed of the AGC circuit by increasing a speed in recharging or discharging the capacitor 7, the gain is largely changed even if the recharging or discharging is performed for a short period of time. Therefore, as shown in FIG. 9, inclination of output amplitude, what is called a sag, to an ideal output waveform appears remarkably with respect to an actual output waveform and the voltage of the AGC filter.
An AGC circuit obtained by further adding an AGC filter to the AGC circuit shown in FIG. 8 has been known as a means for preventing occurrence of the sag. FIG. 10is a block diagram showing an example of another conventional AGC circuit based on a peak detection system. Same reference numerals are provided to the sections in FIG. 10 that perform the same functions as the sections shown in FIG. 8, and to avoid repetition, their explanation is omitted.
The AGC circuit based on a peak detection system shown in FIG. 10 comprises, in addition to the above-described AGC filter (first AGC filter in the figure) 501, a buffer 8, and a second AGC filter 502, and output from the second AGC filter 502 is inputted into the voltage-controlled amplifier 1 as a control voltage.
The buffer 8 inputs a voltage received by the first AGC filter 501 into the second AGC filter 502 as it is, and is formed for targeting mainly impedance conversion. The second AGC filter 502 forms a low-pass filter with a resistor 9 and a capacitor 10 provided therein. A voltage outputted from the first AGC filter is inputted to the resistor 9 via the buffer 8.
Because a high-frequency component is removed from the control voltage outputted from the second AGC filter 502 by a function of low-pass filtering, the control voltage smoothly fluctuates. In accordance with the smooth fluctuation of the control voltage, the gain of the voltage-controlled amplifier 1 is also smoothly changed. Therefore, the above-described sag can be made gradual.
As described above, with the conventional AGC circuit based on a peak detection system shown in FIG. 10, it is possible to make rapid an AGC operation speed and also to suppress occurrence of sag. Although this example shows the system using the buffer 8, a control voltage can directly be inputted into the second AGC filter 502 without the buffer 8 as well, and in this case, the same features can also be obtained.
In the conventional AGC circuit based on a peak detection system, however, there may occur inconvenience because the operation speed is different between the first AGC filter and the second AGC filter. FIG. 11 and FIG. 12 show a relation between input and output when the conventional AGC circuit based on a peak detection system is used, and they are figure to explain the inconvenience mentioned above.
When an amplitude of a modulated signal inputted into the voltage-controlled amplifier 1 becomes sharply small, as the capacitor 7 of the first AGC filter 501 is charged with a small amount of current, an output voltage increases gently, thus a high-frequency component is hardly included in the output voltage.
Therefore, the second AGC filter 502 inputs the output voltage from the first AGC filter 501 into the voltage-controlled amplifier 1 as a control voltage almost as it is, thus an ideal detection output can be obtained even when an amplitude of a modulated signal abruptly decreases as shown in FIG. 11.
On the contrary, when a largely modulated signal is suddenly inputted, a large amount of current is discharged from the first AGC filter 501, so that the output voltage sharply changes rapidly. Accordingly, a large amount of high-frequency component is included in the output voltage, and this high-frequency component is then removed therefrom by the second AGC filter. When amplitude of a modulated signal sharply increases, an overshoot of the detection output is caused for the ideal detection output as shown in the figure.
Since the high-frequency component is removed from the output voltage by the second AGC filter and a delay occurs thereby, there may occur a case where, even when the output voltage from the first AGC filter 501 has reached a desired voltage, an output voltage from the second AGC filter 502 has not yet reached the voltage. Namely, the gain of the voltage-controlled amplifier 1 has not decreased sufficiently enough, so that the first AGC filter 501 keeps on discharging as it is. As the result, overshooting of the detection output, namely excess of the AGC operation occurs, occurs as shown in FIG. 11.
The cause of the excess includes a large amount of discharging current from the first AGC filter 501 and existence of a comparatively large delay because the second AGC filter 502 has a low cut-off frequency as shown in FIG. 12.